1. Field of the Invention
Steel of a type for use in the corrosive environment of hydrogen sulfide, such as line pipe, must have excellent resistance against hydrogen induced cracks (hereinafter called "HIC resistance").
Means to improve HIC resistance must be capable of decreasing inclusions existing in the steel and controlling formation of the same to prevent accumulation of hydrogen in the steel. Accordingly, Ca is sometimes added to the steel in the melting step to control formation of inclusions.
Controlling formation of inclusions by adding Ca is performed to prevent generation of MnS and to form CaS. MnS inclusions become elongated in the direction of rolling and deteriorate HIC resistance. Furthermore, oxide inclusions, for example, Al.sub.2 O.sub.3, which also deteriorate HIC resistance, are converted to, for example, CaO.multidot.Al.sub.2 O.sub.3 to lower its melting point, accumulate, combine and coarsen. This allows the CaO.multidot.Al.sub.2 O.sub.3 to float and separate for easy removal. Thus, clean steel including few inclusions can be manufactured. Therefore, HIC resisting steel has been manufactured by adding Ca in such a manner that the level of sulfur in molten steel is lowered and then Ca is added.
According to the present invention, there is provided a method of manufacturing steel having Ca added thereto that is capable of satisfactorily controlling formation of inclusions and reducing the same.
2. Related Background Art
Hitherto, Ca has been added in a process of manufacturing steel having Ca added thereto by a known method comprising the steps of, in a secondary refining step after molten steel has been refined in a converter, subjecting the molten steel in a ladle to a desulfurization flux process; and continuously adding Ca alloy particles or Ca alloy to the molten steel in the ladle. Another method has been known which comprises the step of continuously or intermittently adding Ca alloy to molten steel in a tundish for use in a continuous casting process.
Although the method in which Ca alloy particles or Ca alloy wires are continuously added in molten steel in a ladle allows a sufficiently long time to be taken to control formation of inclusions, an excessively long time is required to cast the molten steel. As a result, the yield of Ca is generally unsatisfactory and unstable, thus resulting in considerable scattering of the quantity of remaining Ca. Thus, there arise problems in that formation of inclusions cannot satisfactorily be controlled and that the same cannot stably be controlled.
On the other hand, the method in which the Ca alloy is added to molten steel in a tundish exhibits excellent yield of Ca. However, there arises a problem in that the time taken from the addition of the Ca alloy to perform the casting operation is too short to satisfactorily control formation of inclusions.
As a method that is capable of overcoming the foregoing problems, a method has been disclosed in JP56-139613, the title of which is "METHOD OF MANUFACTURING CLEAN STEEL", and in which Ca alloy is added in two steps, namely Ca alloy is added to molten steel in the ladle and to the molten steel in the tundish.
The method disclosed in JP56-139613 will now be described specifically. That method comprises:
(1) a step in which molten steel in a ladle, which has been deoxidized with Al so as to be killed after it has been discharged from a converter, is subjected to a vacuum degasification process, and then Ca is added while stirring the molten steel with Ar gas, followed by continuously stirring the molten steel with Ar gas after Ca has been added; and
(2) a step in which the molten steel is, through a tundish, supplied to a mold so as to be continuously cast in such a manner that Ca alloy or alloy containing Ca is continuously added to the molten steel in the tundish.
The method disclosed in JP56-139613 has as an essential portion that Ca is added after molten steel in the ladle has been subjected to the vacuum degasification process.
The inventors of the invention disclosed in JP56-139613 considered that subjecting molten steel to the vacuum degasification process after Ca has been added to the molten steel in the ladle must be avoided. The reason for this is that evaporation of molten Ca and dissociation of low-melting-point CaO-Al.sub.2 O.sub.3 inclusions causes the melting point of the inclusions to be raised and, thus, the quantity of Ca that must be added to the molten steel in the tundish is enlarged and the cleanability of the steel undesirably deteriorates.
However, the method disclosed in JP56-139613 cannot satisfactorily perform, in the first Ca addition step, deoxidation by means of Ca in such a manner that the Ca alloy added to molten steel in the ladle lowers the melting point of inclusions by converting the Al.sub.2 O.sub.3 inclusions into CaO-Al.sub.2 O.sub.3 and cause the inclusions to float and separate from the molten steel. Thus, the foregoing method suffers from a problem in that the quantity of Ca that must be added to molten steel in the tundish in the second step cannot clearly be calculated. As a result, there arise problems in that oversupply or shortage of Ca inhibits satisfactory control of inclusion formation, excess addition of Ca results in unsatisfactory cost reduction, and excess Ca deteriorates the quality of the steel.